A first aspect of the present invention relates to a method of working a gemstone workpiece, comprising using a rotary working tool in the form of a multiple stone grinding head or bruting crown. For clarity, the gemstone being worked is referred to as the workpiece gemstone or stone, and the stones in the bruting crown are referred to as grinding stones.
The first aspect of the invention is particularly applicable to bruting, i.e. forming a girdle around a gemstone prior to polishing the stone; the girdle can be circular, or non-circular in the case of stones like marquises. However, it is also applicable to coning, i.e. forming the pavillion of the gemstone into a cone, although the tool is still called a bruting crown.
In general terms the invention is applicable to any suitable gemstone, but the invention has been deviated with reference to workpiece diamonds which are worked using diamond. The fundamental problem with diamonds is that no material is harder, and that to work a diamond, in general terms, the relative movement must be on the grain (defined by the crystollographic planes); if the relative movement is more than 7.degree. off the grain, there is no working, and in general one must keep within 5.degree. of the grain.
The precise mechanism which occurs is not known, but the following explains what is observed in practice. During cutting operations, such as bruting or in a coarse polishing process, termed grinding herein, micro-cleaving occurs on cleavage planes and small crystalline grains are abraded away. To achieve this, it is considered normally necessary to have some points on the working tool. However, the operation is relatively fast. During fine polishing, a form of molecular plastic deformation occurs and the only material removed from the workpiece diamond is non-diamond carbon. The operation is slow. A frosted bruted surface indicates grinding and a lustrous surface indicates fine polishing. Both grinding and fine polishing can occur at the same time; for instance a bruted stone often has a squarish girdle, having flattish sides which have been ground down and radiussed corners which have been fine polished.
Damage to the stones can occur if fine polishing commences during bruting; this may happen if the stones have lost their sharp cutting points, and no powder is being formed. If the workpiece is fed at the same rate as for bruting and fine polishing starts to occur, the stone may become chipped or knocked off the dop because fine polishing is much slower than grinding.
For bruting, some modern machines mutually bruts two stones which are rotating in the same direction (so that their peripheries engage in opposite directions) about parallel axes (see for instance GB-A-2 018 173, GB-A-2 082 100 and GB-A-2 200 582). Generally a gemstone is used as a tool to brute a partly processed gemstone to completion, whereupon the tool stone becomes the partly processed gemstone for completion by a new gemstone as the tool. Although this type of bruting is commercially successful, various disadvantages have been identified. One is the formation of squarish bruted girdles, referred to above. Another is that it is difficult to automate the finish as the feed depends on the reduction in diameter not only of the stone being bruted, but also of the other stone. Because of the irregularity of stones (particularly the tool stone), automatic control by controlled feed is not possible. The rate at which the stones are worn down is not predictable, as it is the sum of a known rate of removal (for the partly processed gemstone) and an unknown rate of removal (for the tool stone).
Grinding wheels are used generally in industry with a working face implemented with grit or powder, which may be diamonds; most frequently, the grits or powders are of 4 microns or less, down to below 1 micron. General practice is to use the grinding wheels with very high peripheral speeds. It has been suggested that such grinding wheels can be used for grinding or fine polishing gemstone facets or girdles, though the speeds are still relatively high. One problem is that the wheel wear is heavy so that it is difficult to automate the completion of the operation. There is also a danger that points on the workpiece stone can rip the grinding wheel. It is known to use whole grinding stones in drill bits and in dressing tools.
It is found desirable to provide a bruting tool which has a low and predictable rate of wear, which produces a rate of cutting which is acceptable in a commercial machine, and which allows automatic control of the bruting operation to the necessary degree of accuracy.
The second aspect of the invention relates in general to determining the bruted girdle diameter of a gemstone. GB-A-2 080 712, GA-A-2 200 582 and ZA-A-76/7290 disclose devices and methods for determining the bruted girdle diameter. It is desirable to simplify the methods used.